In Packet Core Networks, PCN, the transmitted user payload will be encapsulated and de-encapsulated by adding and removing GPRS Tunneling Protocol, GTP, header. Two consequent GTP tunnels, between the eNodeB and the Serving Gateway, S-GW, and between the S-GW and the PDN Gateway, P-GW, need to be established to successfully transmit packets between the User Equipment, UE, and the Packet Data Network, PDN, while the S-GW acts as the intermediary between the two tunnels.
When the IP packet of user payload is big enough, the final IP packet size after encapsulation could exceed the allowed value. An example of the allowed value is a Maximum Transmission Unit, MTU, of 1500 bytes. As a result of the MTU limit, the payload has to be fragmented among eNodeB, S-GW and P-GW in the transmission path. The minimum MTU of a layer refers to the size of the largest protocol data unit that the layer can pass onwards. In the PCN, this relates to the transmission capacity between two nodes, such as between the eNodeB and the S-GW.
Currently, eNodeB, S-GW, and P-GW reuse standard IP fragmentation mechanism, i.e., the fragmented packets will be reassembled at IP level, and then processed at user space for GTP tunneling. All of eNodeB, S-GW and P-GW will have to perform this reassembling and fragmentation, if unfortunately large packets, called “jumbo packet” arrives.
Looking at uplink data transmission (data originating from the UE), the first fragmentation begins at the entry of the first tunnel (between the eNodeB and the S-GW). For downlink, data first tunnel would instead be the tunnel between the P-GW and the S-GW. The data for uplink may be fragmented again and sent out at the end of the second tunnel (between S-GW and P-GW). In eNodeB/S-GW/P-GW, each network element will reassemble the IP packet to get the full data and then establish a GTP tunnel to send the data to the next network element. Note that both the IP header and data will be tunneled by GTP. From the fragmentation process, it can be deduced that in the worst condition, the time for fragmentation handling along the transmission path, involving eNodeB/S-GW/P-GW, is:T(eNodeB/P-GW fragmentation)+T(S-GW reassembly)+T(S-GW fragmentation)+T(P-GW/eNodeB reassembly)+T(P-GW/eNodeB fragmentation)
Obviously, it can be seen that too many fragmentation plus reassembly are involved in the GTP tunnel transmission path. As a result, the performance is impacted badly. Besides, when there is large number of fragments in the network, fragmentation and reassembly will consume much effort of kernel IP resources, and do harm to the network throughput.
There is a Change Request, CR, for 29.061 in 3GPP, including a proposal to use MTU discovery to find a minimum MTU along the transmission path. Each network node can use this MTU to detect and make IP fragmentation, as it will avoid extra IP fragmentation along the path. The Mobile Station/User Equipment, or a server in an external IP network, may find out the end-to-end MTU by path MTU discovery and hence fragment correctly already at the source.
However, in the proposal according to the Change Request, IP fragmentation still happens if packet size exceeds MTU. Meanwhile, some upper layer protocol, such as TCP MSS (Maximum Segment Size), provides mechanism for segmentation negotiation. This can prevent IP fragmentation, but it's not common for other upper layer protocols.